The present invention relates to a cathode operable at high temperature and a process for preparing the same. More specifically, the present invention relates to a cathode which is operable at higher temperature (for example, at least 1,400xc2x0 C.) than the operational temperature for impregnated cathodes and which comprises environmentally safe material, and a process for preparing the same.
Conventionally, a cathode as shown in FIGS. 15(a) and (b) has been used for a medium to large electron tube such as a tube for huge power supply equipment. Meanwhile, a cathode shown in FIG. 15(a) has been generally used for a lamp of high power discharge tube, such as a source lamp for photolithography machines. These cathodes, operable even at high temperature of at least 1,400xc2x0 C. where impregnated cathodes are inoperative, comprises a tungsten cathode 21 containing (about 2% by weight of) thorium oxide (ThO2) (hereinafter referred to as thoriated cathode) which is connected to an electrode 20. Recently, impregnated cathodes are gradually applied for medium to large electron tube, since there are improvements in the degree of vacuum inside the tube and change in tube design based on environmental requirement. However, thoriated cathodes are the only practical cathode for the lamp of high power discharge tube, and cannot be easily replaced with the impregnated cathode.
Referring to the thoriated cathode, ThO2 in tungsten W is deoxidized by tungsten or carbon C on the surface of the cathode at about 1,500 to 1,800xc2x0 C., and a Th-W mono atomic layer is formed on the cathode surface. Thereby, work function of about 2.7 eV can be achieved, and electron emission characteristics of about 10 A/cm2 can be obtained under vacuum of 10xe2x88x925 Pa at 2,000xc2x0 C. The fact indicates that the electron emission characteristics is improved by about 1,000 times or more as compared with tungsten cathodes (which have work function of about 4.5 eV). However, since ThO2 contained in the thoriated cathode is a radioactive material, strict management is required for handling. Also, there are potential health and environmental problems. Along with recent environmental approaches, there is a tendency to restrict or stop the use of thorium mainly among its providers, i.e., European countries, indicating another possible problem of lack of stable supply in future.
In addition to the thoriated cathode and the tungsten cathode, there are cathodes having a construction shown in FIG. 16. These are used as high-intensity electron beam source for an electron beam photography machine of an electron microscope or ultra LSI micro processing. The cathode is operable at high temperature and constructed in a way that a lanthanum boride (LaB6) cathode 22 is connected to electrodes 20. The cathode has metallic electrical conductivity and relatively low work function (2.68 eV). The electron emission characteristics of about 20 to 100 A/cm2 can be obtained under vacuum of 10xe2x88x925 Pa at operational temperature of 1,600xc2x0 C. In addition, the cathode has relatively high ion bombardment resistance, and the original electron emission characteristics can be easily recovered even after exposure to atmosphere. However, since LaB6 has monocrystal structure, it is necessary to select most appropriate (100) or (210) crystal plane to draw sufficient electron emission characteristics. Relatively speaking, life time of LaB6 is as short as 500 to 2,000 hours. This is because problems still remain with respect to the stability of LaB6 composition. In other words, though LaB6 is far more stable than other rare earth borides (such as YB6 and GdB6), many report the problems with the stability of surface composition at high temperature. Thus, LaB6 involves disadvantage in difficult handling due to the monocrystal structure and life time due to the stability of compound in itself.
Another but minor example is a zirconium-covered tungsten cathode 23 (monocrystal (100) plane) as shown in FIG. 17. This is partially used for an electron beam photolithography machine for the micro processing of ultra LSI. In the zirconium-covered tungsten cathode, zirconium hydride is thermally decomposed in vacuum and zirconium is adsorbed on the surface of tungsten. By introducing oxygen thereafter, electric dipole moment of a Zrxe2x80x94Oxe2x80x94W layer 24 is formed on the surface. This enables to reduce work function to about 2.4 eV and excellent characteristics can be achieved. As a similar construction, development of Tixe2x80x94Oxe2x80x94W (monocrystal (100) plane) has been reported so far. It is said that the operational temperature is about 1,500xc2x0 C. and life time thereof is 5,000 hours, while vacuum of at least 10xe2x88x927 Pa is required. In any case, there are many problems such as selection of crystalline plane of tungsten monocrystal and practical reproducibility.
As mentioned above, the use of thoriated cathode operative at high temperature involves potential health and environmental problems since it contains radioactive materials. In addition, stable supply of the material is also at stake. On one hand, impregnated cathodes are generally not operable when the temperature is at least 1,400xc2x0 C. And LaB6 or zirconium-covered tungsten cathodes (monocrystal (100) plane) have, on the other hand, problems with handling difficulty such as plane direction adjustment, and stability.
The present invention has been carried out in order to solve the above problems. The object of the present invention is to provide a cathode which is easy to handle and harmless at the same time with a construction which is stable and capable of generating excellent electron emission characteristics even at high temperature of at least 1,400xc2x0 C., and a process for preparing the same.
The cathode of the present invention comprises a polycrystalline substance or a polycrystalline porous substance of high-melting point metal and an emitter material dispersed into the polycrystalline substance or the polycrystalline porous substance in an amount of 0.1 to 30% by weight in the cathode, wherein the emitter material comprises at least one selected from the group consisting of hafnium oxide, zirconium oxide, lanthanum oxide, cerium oxide and titanium oxide.
By adopting this construction, a monatomic layer derived from hafnium oxide, zirconium oxide, lanthanum oxide, cerium oxide and titanium oxide (including Hfxe2x80x94W or the like without oxygen and Hfxe2x80x94Oxe2x80x94W or the like through oxygen) is formed on the surface of high-melting point metal such as tungsten or molybdenum (Mo) at high operational temperature. The monatomic layer is relatively stable at high temperature, reduces work function, and serves as a cathode capable of generating excellent electron emission.
The high-melting point metal material is preferably alloy obtained by adding 0.01 to 1% by weight of Hf, Zr or Ti to tungsten or molybdenum. These added elements act as a reducing agent to improve reducing ability of the high-melting point metal element.
It is preferable to dispose a metal layer of at least one selected from the group consisting of iridium (Ir), ruthenium (Ru), osmium (Os) and rhenium (Re) at least on an electron emission surface of the polycrystalline substance or the polycrystalline porous substance. According to this, work function is further decreased.
It is also preferable to dispose a tungsten carbide layer or a molybdenum carbide layer at least on an electron emission surface of the polycrystalline substance or the polycrystalline porous substance. According to this, work function is further decreased.
Preferably, crystalline grains of the polycrystalline substance or the polycrystalline porous substance are structured fibrously in the same direction. According to this, toughness is improved and processing becomes easier. Furthermore, when carbonization takes place, a carbide layer is formed only on the outermost surface due to this high density construction.
In another embodiment of the present invention, a compound layer of at least one selected from the group consisting of hafnium tungstate, zirconium tungstate, lanthanum tungstate, cerium tungstate and titanium tungstate is disposed on an electron emission surface. According to this construction, hafnium tungstate, for example, will decompose to tungsten and hafnium oxide under cathode operating conditions of high temperature and vacuum. The thus obtained tungsten and hafnium oxide is excellent in homogeneity and reduction effect of tungsten proceeds smoothly, advantageously contributing to long life of the cathode.
The process for preparing a cathode of the present invention is a process in which an emitter material is dispersed in a polycrystalline substance or a polycrystalline porous substance of the high melting point metal material, and the process comprises using, as at least one component of the emitter material, a powdery compound of at least one selected from the group consisting of hafnium tungstate, zirconium tungstate, lanthanum tungstate, cerium tungstate and titanium tungstate. According to this, tungsten and an emitter made of oxide disperse uniformly, and the emitter material can be reduced smoothly.
Another process for preparing a cathode of the present invention comprises mixing, in water or an organic solvent, oxide powder of high-melting point metal material with oxide powder of at least one metal selected from the group consisting of Hf, Zr, La, Ce and Ti, and then calsining and sintering the mixture. According to this, the oxide of high-melting point metal is reduced, and then, it is possible to disperse an emitter material containing at least one selected from the group consisting of hafnium oxide, zirconium oxide, lanthanum oxide, cerium oxide and titanium oxide to the high-melting point metal material.
Another process for preparing a cathode of the present invention comprises mixing a solution obtained by dissolving, in water or an organic solvent, a nitrate of at least one metal selected from the group consisting of Hf, Zr, La, Ce and Ti with oxide powder of high-melting point metal material, and then calsining the mixture. According to this, the oxide of high-melting point metal is reduced and the nitrate is decomposed as well. And then, it is possible to disperse an emitter material containing at least one selected from the group consisting of hafnium oxide, zirconium oxide, lanthanum oxide, cerium oxide and titanium oxide to the high-melting point metal material.
Another process for preparing a cathode of the present invention comprises impregnating a solution obtained by dissolving, in an organic solvent, an alcoxide of at least one metal selected from the group consisting of Hf, Zr, La, Ce and Ti into a porous high-melting point metal material under reduced pressure, and then calsining the mixture. According to this, the alcoxide is decomposed, and then it is possible to disperse an emitter material containing at least one selected from the group consisting of hafnium oxide, zirconium oxide, lanthanum oxide, cerium oxide and titanium oxide to the porous, high-melting point metal material.
Another process for preparing a cathode of the present invention comprises covering, on powder of high-melting point metal, an alcoxide of at least one metal selected from the group consisting of Hf, Zr, La, Ce and Ti, and then calsining the mixture. According to this, the alcoxide is decomposed into an oxide, and the high-melting point metal powder covered with the oxide is formed. As a result, it is possible to disperse an emitter material containing at least one selected from the group consisting of hafnium oxide, zirconium oxide, lanthanum oxide, cerium oxide and titanium oxide to the high-melting point metal material.
It is preferable to pulverize a solid material formed by covering the oxide on powder of the high-melting point metal through the calsining step, to mix it with another powder of high-melting point metal, and then to sinter the mixture. According to this, mechanical strength of molded articles can be improved.
Preferably, the calsining/sintering step of the preparation process is carried out at temperature such that the emitter material is not deoxidized. According to this, it is possible to inhibit vain evaporation of the emitter material such as hafnium oxide and generation of final product.
Preferably, the above each process further comprises a step for drawing the high-melting point metal material, into which the emitter material is dispersed, by swaging in hydrogen gas. According to this, crystalline grains of the high-melting point metal can be structured fibrously in the same direction, and therefore, toughness is improved as well as excellent processability is achieved.
It is preferable to form a tungsten carbide layer or a molybdenum carbide layer at least on the electron emission surface of the cathode after fibrous structure is formed. According to this, a favorable construction can be obtained since carbonization takes place particularly only on the outermost surface not in the inside.